Tuneable phase shfter and/or attenuator

ABSTRACT

The invention relates to a tuneable phase shifter and/or attenuator comprising a waveguide having a channel and a piece of photo-responsive material ( 18 ) disposed within the waveguide along an internal wall of said channel, a light source disposed outside the waveguide to emit light through an aperture ( 30 ) of said internal wall to impinge on at least part of an outside surface of said piece of photo-responsive material ( 18 ).

The present invention relates to a phase shifter and/or attenuator andin particular to an optically tuneable phase shifter and/or attenuatorcapable of operating in the microwave, millimetre and sub-millimetrewave spectrum. The phase shifter and/or amplitude attenuator may be usedin a wide range of applications including, but not limited to,phase-shift-keying circuitry, terahertz imaging, transceivers andphased-array antennas.

As far as the sub-millimeter range is concerned, terahertz technologybeen primarily been used in the fields of terrestrial and astronomy andearth observation. However, many materials that are opaque in theoptical and infrared regions are transparent to terahertz waves (0.1 THzto 10 THz). Applications for terahertz technology have thus recentlyexpanded to include areas such as aerial navigation where terahertzwaves are able to penetrate clouds and fog, medical imaging where bodytissue can be examined without using potentially harmful ionisingradiation, and non-invasive security systems for use at airports andports in which the terahertz waves are able to pass through clothing andmaterials normally opaque to infrared.

Owing to the sub-millimetre wavelengths of terahertz waves, the requireddimensions and accuracy of components such as antennas, waveguides,lenses, mirrors etc. make fabrication difficult and costly usingconventional manufacturing techniques.

In the millimetre waveband, ferroelectric phase shifters are oftenemployed in which the phase of the signal is shifted by varying thepermitivity of the ferroelectric material by means of an appliedelectric field. However, ferroelectric phase shifters suffer fromsubstantial power losses, signal distortions and noise, and offer onlydiscrete steps.

An optically activated waveguide type phase shifter and/or attenuatorhas been disclosed in U.S. Pat. No. 5,099,214 (ROSEN et al.). Thisdevice comprises a semiconductor slab 24 that is attached to an insidewall 12 of waveguide and which receives light from an illuminationsource 30 disposed in an aperture of an inside wall 14 opposite insidewall 12. In U.S. Pat. No. 4,263,570 (DE FONZO), a piece 20 ofsemiconductor material is attached to an inside wall 22 of a waveguideand an inside surface of said piece is lit from outside by a lightsource 12 through an aperture 30 in a wall 28 opposite inside wall 22.

In these prior art documents, where illumination is from the oppositewaveguide wall, a lossy resistive layer forms inside the waveguide at adistance from the inside wall that is equal to the thickness of thesemiconductor piece or slab, which means that the insertion losses willbe always high, and that a high level of light is necessary to obtain asignificative phase shift or attenuation. Namely, this light levelshould be generally high enough to generate a high density of carriersto place the photo-sensitive material (Si) in a metallic orsemi-metallic state.

It is therefore an object of the present invention to provide a tuneablephase shifter and/or attenuator capable of operating at microwave,millimetric and/or sub-millimetric wavelengths with an improvedtuneability. According to the invention, this is obtained by apositioning of a light source and/or a photo-responsive material spacedrelatively to the waveguide, and by providing a modification of thecarrier concentration within a photo-responsive material by theillumination of light.

According to a first aspect, the present invention provides a tuneablephase shifter and/or attenuator comprising a waveguide having a channeland a photo-responsive material disposed within the waveguide along aninternal wall of said channel, a light source disposed outside the waveguide to emit light through an aperture of said internal wall to impingeon at least part of an outside surface of said photo-responsive materialAccording to this first aspect, the phase is modified by changing theeffective width of the waveguide, without changing the mode ofpropagation.

The photo-responsive material preferably has a high electricalresistivity. The surface of the photo-responsive material facing theaperture can be pacified, e.g. by oxidation.

The phase shifter may also include a plurality of metal strips whichextend across the surface of the photo-responsive material facing theaperture. The purpose of this metallic grid is to avoid the internalwave travelling inside the waveguide being radiated outside it and alsoto allow light (smaller wavelength), to enter the waveguide. The size ofthe grid depends on the frequency of the radiation propagated by thewaveguide.

In U.S. Pat. No. 5,099,214, it has been also suggested to space slab 24off wall 12 by a distance x that may be such that slab 24 is centeredalong distance n, n designating the waveguide width.

However, this positioning of the slab inside the waveguide and spacedfrom the wall is even less favourable relative to insertion losses. Theinventors have identified that there is another phenomenon than changingthe effective waveguide width through the creation of a quasi metallicstate in the semiconductor namely varying the imaginary part of thedielectric constant of the semiconductor by illumination so that otherwaveguide modes are able to propagate that would not normally bepresent.

According to a second aspect, the present invention provides a tuneablephase shifter and/or attenuator comprising a waveguide having a channeland a piece of photo-responsive material disposed within the waveguideand spaced from an internal wall of said channel, and a light source toemit light to impinge on at least part of a surface of saidphoto-responsive material, the light source being adjustable inintensity and/or illumination length to generate in the photo-responsivematerial a carrier concentration between 10¹² cm⁻³ and 10¹⁶ cm⁻³, tomodify the real and imaginary part of the dielectric constant of thephoto-responsive material to generate at least one mode that has part ofits field inside the photo-responsive material layer and part of itsfield in the waveguide whereby a phase shifter and/or attenuator that isdependant on the light illumination (in intensity and/or length) isgenerated over a frequency range.

The phase light is obtained by changing the mode of propagation. Movingthe semiconductor layer away from the waveguide wall, allows higherorder modes to propagate over the said frequency range and these havegreatly different effective guide wavelengths and phase.

The photo-responsive material may be photo-conductive material such as asemiconductor for example Si, GaAs or Ge, whether intrinsic or doped.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a tuneable phase shifteror tuneable attenuator in waveguide technology in accordance with thepresent invention;

FIG. 2 is a schematic cross-sectional view of a tuneable phase shifteror tuneable attenuator in waveguide technology in accordance with thepresent invention taken along the line A-A in FIG. 1;

FIG. 3 is a schematic cross-sectional view of radiation propagatingthrough a tuneable phase shifter or tuneable attenuator in waveguidetechnology in accordance with the present invention; and

FIG. 4 is a further schematic cross-sectional view of radiationpropagating through a tuneable phase shifter or tuneable attenuator inwaveguide technology in accordance with the present invention.

FIG. 5 illustrates the Absorbtion coefficient a of Si (in mm⁻¹) versusphoton wavelength (in nanometers).

FIG. 6 illustrates the refraction index of Si versus photon wavelengthin nanometers, FIG. 7 the percentage of light reflected transmitted andabsorbed by Si versus photon wavelength in nanometers (curves I, II andIII respectively), and FIG. 8 the percentage of light absorbed by Siversus photon wavelength (in nanometers) for three different Si waferthicknesses 50μ (I), 100μ (II) and 600μ (III).

FIGS. 9 and 10 show the dielectric constant and tan δ of Si respectivelyat 40 GHz and 250 Hz.

FIG. 11 shows the wavelength (in millimetres) inside a WR-28 waveguideversus frequency in the Ka band and versus a change in the parameter a.

FIGS. 12 a and 12 b show an inhomogeneously filled waveguide with adielectric piece of thickness t in a wall thereof and the fundamentalmode TE₁₀ therein.

FIG. 13 shows curves of the wavelength (in millimeters) as a function offrequency (GHz) inside a WR-28 waveguide with a 300μ thick piece of Siin a wall thereof under different light conditions.

FIG. 14 shows curves of the wavelengths (in millimeters) as a functionof frequency (GHz) for a WR-28 waveguide with a piece of Si in a wallthereof with different thicknesses 300μ (I), 500μ (II), 1000μ (III andIV), and two different light conditions for the thickness of 1000μ.

FIGS. 15 and 16 a and 16 b show an inhomogeneously filled WR-28waveguide with an inside dielectric piece spaced from a wall of thewaveguide for resultant modes respectively TE₂₀ mode, TE₁₀ mode and TE₁₁mode; these modes are not equal to the modes of a conventionalrectangular waveguide.

FIG. 17 representes the wavelength (in millimeters) of the propagativemodes inside a WR-28 waveguide with a 300μ thick silicon dark piecesspaced 0.85 mm from a wall of a waveguide for TE₁₀ and TE₂₀ modes anddifferent illumination levels corresponding to different densities ofcarriers inside the silicon piece,

and FIG. 18 illustrates propagation at different frequencies and undersix different illumination states of a WR-28 waveguide with a piece ofSi spaced 0.85 mm from a wall of the waveguide.

The tuneable phase shifter 10 illustrated in FIGS. 1 and 2 comprises awaveguide 11 having a central channel 12 which extends the length of thewaveguide 11 and an aperture formed in a side 13 of the waveguide 11.The tuneable phase shifter 10 may further comprise a metallic grid 20 toavoid radiation of the microwave, mm-wave or submm-wave inside thewaveguide to be lost outside the waveguide system.

A photo-responsive layer 18 is disposed within the channel 12 of thewaveguide 11 so as to extend substantially across the aperture. Anadjustable irradiation source of light 14 emits light at a certain partof the spectra where the photo responsive material inside the waveguideabsorbs it better (infrared, visible, ultraviolet . . . ). Source oflight 14 is located outside the waveguide such that irradiatingradiation from the source 14 is incident upon an area of thephoto-responsive layer 18 exposed by the aperture 30 formed in a side 13of the waveguide 11. The photoconductive material is placed directlyagainst the waveguide wall and is illuminated through the wall againstit is placed. If the intensity of light is sufficient, a quasi-metalliclayer is formed at the waveguide wall/photo-responsive material boundarywhich is closest to the waveguide wall. This layer changes the effectivewidth of the waveguide which results in a change in effective guidewavelength and hence phase. As the thickness of the quasi-metallic layer26 is depended on the light intensity, so is the phase shift.

The photo-responsive layer 18 may be of semiconductive material, e.g.Si, AsGa, Ge.

The waveguide 11 comprises a silicon or metallic body 15 having acentral channel 12 substantially rectangular in cross-section extendingthe length of the silicon body 15. The width and height of the channel12 may be as is conventionally employed in rectangular waveguideconstruction. However, the dimensions of the silicon body 15 may beadjusted according to preference.

The inner surfaces 16 of the silicon body 15 may be coated with ametallic film 17, preferably using for example vacuum deposition andelectroplating techniques. Suitable metals for coating the silicon body15 include, but are not limited to, nickel, copper, brass, chromium,silver and gold. The metal coating 17 acts to reflect radiationpropagating along the length of the channel 12. Accordingly, the coating17 may comprise any material which serves to reflect radiation.

Alternatively, a completely metallic waveguide made for example by amilling machine may be used.

A construction of metallised silicon waveguides for terahertzapplications using micromachining techniques is known and is describedfor example in “Silicon Micromachined Waveguides for Millimeter andSubmillimeter Wavelengths”, Yap et al., Symposium Proceedings: ThirdInternational Symposium on Space Terahertz Technology, Ann Arbor, Ml,pp. 316-323, March 1992 and “Micromachining for Terahertz Applications”,Lubecke et al., IEEE Trans. Microwave Theory Tech., Vol. 46, pp.1821-1831, Nov. 1998.

The aperture formed in the side 13 of the waveguide 11 extends throughthe silicon body 15 and the metal coating 17 on one of the longer sidesof the waveguide 11. The aperture may be rectangular in shape and with awidth substantially similar to the width of the channel 12. The lengthof the aperture is characterised by the desired degree of phase shiftingat the frequency of operation. Generally speaking, the longer the lengthof the aperture (or rather the longer the exposed region of thephoto-responsive reflector 18), the greater the degree of phase shiftingand/or attenuation.

The semi-conductor layer 18 may be associated with a plurality ofreflective elements 20. The layer of photo-responsive semi-conductorlayer 18 has for example an upper 21 and lower 22 surface substantiallyrectangular in shape. The width of the layer 18 may be substantiallysimilar to the width of the channel 12, whilst the length of the layer18 is preferably longer than the length of the aperture formed on theside 13 of the waveguide 11. Preferably the length of the layer 18 isonly slightly longer than that of the aperture. The layer 18 is securedwithin the channel 12 of the waveguide 11 such that the layer 18 extendssubstantially across the aperture formed in the side 13 of the waveguide11. The layer of photo-responsive material 18 is secured to a wall 23 ofthe channel 12 for example by a thin layer of adhesive applied at theends 24,25 of the layer 18 extending beyond the length of the aperture.Alternatively, if the waveguide is made of metallised silicon, layer 18may be integral with the waveguide.

The photo-responsive material 18 may be photo-conductive preferablyconsists substantially of intrinsic silicon. However, alternativephoto-responsive materials which may be used include, but are notlimited to, GaAs and Ge.

When the optical radiation is incident upon the exposed surface 21 ofthe photo-responsive layer 18, photo-excited carriers are created at aregion near the surface 21. Accordingly, the dielectric constant of thephoto-responsive material 18 in this region changes; generally referredto as photo-induced reflectivity. The reflectivity of the irradiatedsurface 21 of the photo-responsive material 18 can even be renderedsimilar to that of a metal in dependence upon the intensity of theincident optical radiation, but with this device it is sufficient tohave a small increase of the real part of the dielectric constantassociated with a large increase of the imaginary part of the dielectricconstant. At this point, the photo-responsive material 18 can beregarded as having a separate photo-induced resistive layer (referencenumeral 26 in FIG. 4), but for a thin layer, the effect of the light isto change the dielectric properties of the material in depth, i.e.essentially the imaginary part of the dielectric constant in all thethickness.

Whilst the photo-responsive material 18 is generally transparent to theradiation propagating along the channel 12 of the waveguide 11, somepower loss of the signal will occur. Accordingly, the thickness of thelayer of photo-responsive material 18 may be for example between 60 and100 μm. A higher thickness up to about 1000 μm may be used. Moreover,the photo-responsive material 18 is preferably silicon.

The lifetime of the photo-excited carriers are determined primarily bytheir mobility and the availability of recombination sites in thelattice of the photo-responsive material 18. By increasing the lifetimeof the carriers, the lifetime of the photo-induced reflective layer canbe extended. Accordingly, the irradiation delivered by the source 14 maybe delivered over shorter periods of time. Not only does this reduce theamount of power consumed by the irradiation source but it also preventsthe photo-responsive material 18 from reaching potentially damagingtemperatures which can arise from continuous irradiation. In order toincrease the lifetime of the carriers, the photo-responsive layer 18preferably has a high electrical resistivity (>1 kΩcm⁻²). Thephoto-responsive layer 18 may consist of silicon having an electricalresistivity for example between 4 and 10 kΩcm⁻².

Moreover, the lifetime of the carriers can be further increased forexample by pacifying the irradiated surface 21 of the photo-responsivematerial 19. The surface 21 of the photo-responsive layer 18 offers alarge number of recombination sites. By pacifying the irradiated surface21, the number of recombination sites available to the carriers issignificantly reduced. The uppermost surface 21 of the photo-responsivematerial is therefore preferably oxidised. Even with oxidation, however,the number of recombination sites remains sufficiently high tosignificantly affect the mobility of carriers. It has been found,however, that applying a coating of an adhesive such as an epoxy resinto the oxidised surface of the photo-responsive material cansignificantly increase carrier lifetime.

In having a photo-responsive layer 18 comprising essentially of highresistance silicon for example with a resistivity of between 4 and 10kΩcm⁻² and an oxidised upper surface coated in an epoxy resin, thelifetime of the photo-induced carriers and thus the photo-inducedreflective layer is substantially increased.

Accordingly, phase shifting may be achieved and maintained withrelatively low intensity irradiation. However, in extending the lifetimeof the photo-induced carriers, the response time of the phase shifter isincreased.

It will, however, be appreciated that fast response times can beachieved by having a photo-responsive material in which the lifetime ofthe photo-induced carriers is relatively short. This may be achieved,for example, by having a photo-responsive layer of low resistance andwhose surfaces have not been pacified.

The plurality of reflective elements 20 are formed on the uppermostsurface 21 of the photo-responsive material 18 in the region defined bythe aperture on the side 13 of the waveguide 11. The reflective elements20 are preferably strips of reflecting material. Accordingly, thereflective elements 20 are strips of metal, that may be arranged as agrid. they allow that most part of light entering the photoresponsivematerial. Again, suitable metals include, but are not limited to,nickel, copper, brass, chromium, silver and gold. The strips arepreferably aligned on the surface 21 of the photo-responsive material 18so as to extend substantially parallel to the width of the channel 12and thus perpendicular to the length of the channel 12. The length ofthe strips may be at least the width of the channel 12 and preferablyextend across the full width of the photo-responsive material 18. Thestrips are evenly spaced (or tapered) along the length of thephoto-responsive material 18 and cover preferably less than 50% of theregion of the surface 21 revealed by the aperture 30. The width andseparation of the strips is preferably no greater than 1 mm (this ofcourse depends on frequency of operation). The strips should be of athickness suitable for total reflection of incident radiation withoutany substantial loss. The strips may be applied, for example, byapplying a mask to the surface 21 of the photo-responsive material 19and depositing a metal film using vapour deposition.

The irradiation source 14 may be any source capable of generatingphoto-induced carriers reflectivity in the layer 18 of photo-responsivematerial and is preferably a commercially-available laser or LED arrayhaving a visible or near-infrared wavelength, (in fact having the bestfrequency spectra for absortion by the photo responsive material used).The power required of the source 14 will depend upon, among otherthings, the type of photo-responsive material 18 and the degree of phaseshifting or attenuation required.

An electronic circuit can control the degree of phase shifting orattenuation by means of the illumination of the photoresponsivematerial.

Referring now to FIG. 3, radiation propagating along the length of thechannel 12 of the waveguide 11 is reflected internally by the surfacesof the metal coating 17. When the radiation is incident upon thephoto-responsive material 18, the radiation propagates a little insideit due to its reduced dielectric constant. Upon reaching the uppermostsurface 21 of the layer of photo-responsive material 18, a proportion ofthe radiation is reflected back towards the channel 12 by the pluralityof reflective elements 20. A small fraction of the radiation istransmitted into the air (indicated by a broken line) and thus exits thewaveguide 11. Owing to the angle of incidence of the propagatingradiation with respect to the photo-responsive material 18, no internalreflection occurs within the photo-responsive material 18. Accordingly,the radiation reflected by the reflective elements 20 propagates backthrough the photo-responsive material 18 and into the channel 12. Thepropagating radiation may be incident upon the photo-responsive material18 more than once, according to the length of the reflector 18, beforeit continues propagating along with length of the channel 12 of thewaveguide 11.

FIG. 4 illustrates the situation whereupon irradiating radiationdelivered by the irradiation source 14 is incident upon thephoto-responsive reflector 18. The irradiating radiation generatescarriers in the photo-sensitive material and causes a photo-inducedresistivity in photo-responsive material 18. The effective thickness ordepth of the photo-induced resistive layer 26 will depend upon thewavelength and intensity of the irradiating radiation incident upon thephoto-responsive material 18. When the radiation propagating along thechannel 12 of the waveguide 11 is incident upon the photo-responsivelayer 18, the radiation propagates through the photo-responsive material18 only so far as the photo-induced reflective layer 26. Upon reachingthe photo-induced resistive layer 26, the propagating radiation isreflected back towards the channel 12.

The photo-induced lossy material in layer 18 changes the modalpropagation in the waveguide so that no field will enter the lossyphotoilluminated material but the change in the fundamental mode of thatnew waveguide will effectively change the phase. The propagatingradiation now has a phase (or amplitude) that is substantially differentto radiation propagating along the waveguide 11 in the absence of thephoto-sensitive layer 18. Furthermore, phase shifting will occur everytime the propagating radiation is incident upon the photo-responsivelayer 18. Accordingly, the length of the photo-responsive layer 18 thatis illuminated will also determine the degree of phase shifting. Thisillumination length may be adjustable to adjust phase shift and/orattenuation. As the changes in the modal propagation in the waveguideare determined by the intensity and wavelength characteristics of theirradiating radiation, the degree of phase shifting can accordingly becontrolled by varying the intensity and/or wavelength of the irradiatingradiation delivered by the source 14.

In the device shown in FIGS. 1 to 4, the silicon is illuminated on itsface adjacent to the waveguide wall. This is important as the electricfield in a rectangular waveguide with a semi-conductor inside (placedclose to the wall or slightly spaced therefrom) is highest in the middleof the guide and zero at the edge, therefore a lossy material placedfurther towards the centre of the waveguide will absorb more energy thanif it were placed at the edge. For a phase shifter the most desirablefeatures is low insertion loss and large phase shift for small powerrequirement. When the phase shifter is illuminated at low light levelsphoto carriers are generated changing the resistivity of the material,however, also the imaginary part of the dielectric constant is varied.As the light intensity is increased eventually the silicon takes onmetallic properties. In order to achieve a “quasi metallic layer” withinthe silicon there must be a high density of carriers 10¹⁸-10²¹carriers/cm³ It is important to note, however, that this quasi metallicstate is not an abrupt change from high resistivity to low resistivitybut one that varies exponentially between the each extreme. On one sideof the region (the one that is illuminated) there is a nearly metalstate, the other has a high resistivity state and in between a lossyresistive state. It is this region within the silicon that causes themajority of the insertion loss. This lossy layer will always be on theopposite side of the quasi metal state region than the side thereof thatis being illuminated as the light is decaying exponentially throughoutthe thickness of the silicon. When as in the present invention, thesilicon layer adjacent the waveguide wall is illuminated from theoutside, it starts to form first at the outside of the waveguide, hencethe insertion loss is kept to a minimum. At lower light intensity, thelossy resistive region will be also at the outside of the material 18.In the prior art patents (U.S. Pat. No. 4,263,570 and U.S. Pat. No.5,099,214) where illumination is from the opposite waveguide wall, thelossy layer forms first inside the waveguide at a distance from thewaveguide wall that is equal to the thickness of silicon material 18.This is a fundamental difference and will mean that the insertion losswill always be higher. In addition, this position is fixed physicallywith respect to the waveguide wall. This means that the any resistivityvariation within the silicon will occur between the innermost edge ofthe silicon and the waveguide wall. Consequently it will have arelatively small effect with respect to changing the effective width ofthe waveguide. With an illumination from the outside as in the presentdevice, the opposite is true.

The dimensions of the channel 12 of the waveguide 11, the size andcharacteristics of the photo-responsive reflector 18 and the size of theaperture formed on the side 13 of the waveguide 11 may all be tailoredto suit the desired performance of the phase shifter 10. An example ofthe dimensions that might be used for phase shifting terahertzfrequencies is now described. The width and height of the channel 12 ispreferably around 1.5 mm and 0.75 mm respectively. This provides awaveguide cut-off frequency of around 0.1 THz. Accordingly, the siliconwafer used to construct the silicon body 15 has a thickness of around0.75 mm. The metal coating 17 is preferably of the order of 500 nm. Thewidth of the aperture 30 formed on the side 13 of the waveguide is alsopreferably 0.75 mm. The length of the aperture 30 is preferably around 2cm. The layer of photo-responsive material 19 preferably has a width,length and thickness of around 0.75 mm, 2.5 cm and 70 μm respectivelyand has an oxidation layer on the uppermost surface 21 typically oraround 10-50 nm. Each reflecting element preferably has a width, lengthand thickness of around 0.5 mm, 0.75 mm and 500 nm respectively. Thespacing between reflecting elements is preferably 0.5 mm.

Whilst the embodiment described above comprises a waveguide having asingle aperture and a single photo-responsive layer 18 extending acrossthe aperture, it will be appreciated that two apertures may be formed onopposing sides of the waveguide 11. Two or more photo-responsive layerswould then be employed and the degree of phase shifting or attenuationachievable may be doubled, tripled or quadrupled. It will be appreciatedthat the same technical effect might be achieved by doubling the lengthof the single aperture and photo-responsive reflector 18. Nevertheless,a phase shifter comprising two or more apertures 30 and two or morephoto-responsive layers 18 might be considered when the size, and inparticular the length, of the phase shifter is a serious consideration.

It will be appreciated that the plurality of reflecting elements 20 maybe omitted. In this situation, some form of irradiating radiation mustbe delivered to the photo-responsive reflector 18 such that aphoto-induced reflective layer 26 is continuously present. For example,the irradiation source 14 may continuously irradiate thephoto-responsive reflector 18 with radiation. Alternatively, theirradiation source 14 may deliver pulsed, high intensity irradiation.

Rather than forming a plurality of reflective elements 20 on the surface21 of the photo-responsive material 18 facing the aperture, thereflective elements 20 could be formed on a separate element such as aglass plate. The glass plate could then be placed within the aperture soas to rest on top of the photo-responsive material 18.

The phase shifter 10 may also comprise an attenuator, such as a variableoptical attenuator, to compensate for variations in the amplitude of thepropagating radiation with phase shift, or a simple tuneable attenuator,not necessarily adjoining to the phase shifting device. Moreover, bothphase and amplitude modulation of a signal is then possible.

Signals at millimetre wavelengths require a waveguide having largerdimensions than that for terahertz (sub-millimetre) frequencies.Accordingly, the degree of possible phase shifting is reduced owing tothe reduced ratio of the photo-induced layer thickness with respect tothe waveguide height. However, this reduction in phase shifting can becompensated by having a photo-responsive reflector 18 greater in length.

As the photo-responsive material 18 is generally transparent to thepropagating signal, signal distortion and power loss is generally low incomparison to ferroelectric phase shifters.

The following relates to the advantage obtained for a phase shifter fromthe optical properties of silicon which, as been identified by theinventors, allows a change in the complex relative permitivity of thesilicon as it is illuminated by a source of light in infraredwavelengths.

Illumination of silicon by means of a near-infrared/visible light-sourceproduces the generation of electron-hole pairs, thus producing a plasma.This plasma is directly dependant on the intensity and wavelength of theincident light.

If we assume normal incidence of the light to the silicon wafer, theformulas that explain the properties of the material are as follows:

The amount of light reflected in an interface air-silicon is:$R_{1} = \frac{\left. {\left( {n_{r} - 1} \right)^{2} + n_{i}^{2}} \right)}{\left( {n_{r} + 1} \right)^{2} + n_{i}^{2}}$where n=n, +j·n_(i) and n is the refraction index of the silicon.

For absorption coefficient values greater than zero, the percentage R oftotal light reflected can be determined using the following equation:R≈R ₁+(1−R ₁)·R ₁ ·e ^(−α·2·t)−(1−R ₁)·R ₁ ² ·e ^(−α·2·t)+(1−R ₁)·R ₁ ³·e ^(−α·4·t)−(1−R ₁)·R ₁ ⁴ ·e ^(−α·4·t)+ . . .where the α coefficient is the absorption coefficient of the silicon andit is dependent on the light wavelength, see FIG. 5. And t is thethickness of the silicon wafer.

Each term in the infinite series is associated with the successivereflections as the light bounces between the surfaces of the siliconwafer. Similarly, the percent transmission T can be determined using thefollowing equation:T≈(1−R ₁)·e ^(−α·t)−(1−R ₁)·R ₁ · ^(−α·t)+(1−R ₁)·R ₁ ² ·e^(−α·3·t)−(1−R ₁)·R ₁ ³ ·e ^(−α3·t)+ . . .

-   -   where the percent absorbed light A is given by:        A≈1−(R+T)

There are essentially two regions of strong optical absorption inSilicon. FIG. 5 shows the absorption coefficient versus photonwavelength for the visible-FIR and IR regions respectively. For photonenergies equal-to-or-greater-than the energy gap, normal opticalabsorption with the generation of free carriers occurs.

In FIG. 6, a plot of the refraction index of silicon material isdepicted against wavelength (in nanometers). The refraction index hasits maximum at the violet color of the spectrum, this means thatviolet-blue light is reflected by silicon stronger than other visiblecolors so we see this material as violet-blue coloured.

In FIG. 7 we can see the amount of light power absorbed, reflected andtransmitted by a silicon wafer of 600 μm thickness. The maximumabsortion occurs for red color visible light and near infraredwavelengths.

Also in FIG. 8, a comparison of three different thicknesses wafers isdepicted in terms of light power absorbed by the material, to illustratethe percentage of light absorbed by silicon versus photon wavelength (innanometers).

The semiconductor complex relative permittivity containing electron-holepairs is expressed as a sum of two, electron (e) and holes (h) dependantterms:$\varepsilon_{r}^{Si} = {\varepsilon_{u} - {\sum\limits_{{i = e},h}\quad{\frac{{\overset{\_}{\omega}}_{pl}^{2}}{\left( {2 \cdot \pi \cdot f} \right)^{2} + v_{i}^{2}} \cdot \left( {1 + {j \cdot \frac{v_{l}}{2 \cdot \pi \cdot f}}} \right)}}}$where ω_(pi) ²=(N·q²/ε₀·m_(i)) is the plasma angular frequency,ε_(u)=11.8 is the dark dielectric constant of silicon, ν₁ is thecollision angular frequency, m_(i) is the effective mass of the carrier,q is the electronic charge and ε₀ is the permittivity of free space.

For computation reasons: ε₀=8.854·10⁻¹² F·m⁻¹, ν_(e)=4.53·10¹² s⁻¹,ν_(h)=7.71·10¹² s⁻¹, m_(e)=^(0.259)·m₀, m_(h)=0.38·m₀, m₀=9.107·10⁻²⁸ gis the free electronic mass and N is the number of carriers generated inthe plasma.

The dielectric constant of a material is defined as a real and animaginary part. The relation between the real and the imaginary part iswhat we call the tan(δ) of a material. This important material parameteris directly related with the losses of that material when anelectromagnetic wave passes through it.ε=ε′+j·ε″ tan(δ)=ε″/ε′

In the following figures, a plot of the dielectric constant and thetan(δ) of silicon at different frequencies respectively 40 GHz and 250GHz is depicted against the carrier concentration, N between 10¹⁰ and10²⁰/cm³.

For example, it can be seen in FIG. 9 that at a carrier concentration of10¹⁷ cm⁻³, the real part of the dielectric constant of the silicon at 40GHz is 85.6 and at N=10¹⁸ cm⁻³ is 750 where the silicon has a reallyhigh dielectric constant. At N above 10¹⁷ cm⁻³, the real and imaginarypart of the dielectric constant of the silicon increase with the sameslope, so the tan(δ) becomes constant.

At no light condition, the amount of carriers in the silicon is around10¹⁰ cm⁻³ where the tan(δ) is around 10⁻⁴ at 40 GHz. But as the carrierconcentration increases with light, the silicon becomes a very lossymaterial maintaining its dielectric constant quite stable. As it will beseen in the following passages of the description, it is interesting forphase shift to change the dielectric constant of silicon material toaffect the propagation characteristics of electromagnetic waves, ratherthan changing the losses of the material which will attenuate the waveand which is interesting for the attenuator function of the device. So acertain amount of light per area is required.

In FIG. 10 it can be seen that at higher mm-wave frequencies, (250 GHz),the real part of the dielectric constant of the material behaves exactlyas at 40 GHz, but the imaginary part is lower, but increases with lightwith the same slope, so in fact, the losses are lower at higher mm-wavefrequencies.

From the understanding of the previous properties, it can be said thatchanges in the dielectric material properties of silicon by means of anoptical source of variable intensity can be achieved. This propertyopens a new field of applications to design and manufacture a widevariety of components at mm-wave frequencies by means ofphotoillumination. We assume in our finite element calculations by meansof Ansoft-HFSS that the plasma thickness remains constant while theplasma density varies in this thickness with intensity of applied light.

The main reason of this study is to design, manufacture and measure aphase shifter for rectangular waveguide technology. The tuneable phaseshifter has to achieve a phase shift with high accuracy and as lowlosses as possible. A best mode is a tuneable shifter with a 360° phaseshift. The main idea of this concept is placing a piece of siliconinside the rectangular waveguide and changing its dielectric propertiesby means of appropriate conditions of photoillumination. If a certainsize piece of silicon is placed inside a rectangular waveguide and isilluminated, it changes the propagation characteristics of the waveguideand the transmision characteristics of the waveguide.

The illumination may be performed by means of a metallic grid in one ofthe walls of the waveguide so that it is transparent for light and“metallic” for mm-waves so that the characteristics of the rectangularguide do not change.

Also, a certain amount of light required to perform a change in thepropagation properties of the waveguide with a silicon piece inside. Infact, it easy to check that as the wavelength increases, the amount oflight per unit area will be lower, because the silicon piece needed toperform the change will be smaller. In fact, if we increase thefrequency by a factor of 10, the amount of light per unit area requiredwill decrease by a factor of 100.

For ease of manufacture and measurement reasons the design given asexample was prepared in Ka band for WR-28 standard waveguide. Thedimensions of this waveguide are a=7.1 mm and b=3.6 mm, and in FIG. 11it can be seen the wavelength inside this waveguide against frequency.Also in FIG. 11, we can see the effects on the wavelength (in mm) insidea WR-28 waveguide of a change of its parameter a from 7.1 mm to 5 mm

The wavelength inside a rectangular waveguide is defined by:$\lambda_{g} = \frac{\lambda_{0}}{\sqrt{1 - \left( \frac{\lambda_{0}}{2\quad a} \right)^{2}}}$where λ₀ is the free space wavelength and a is the longest dimension ofa rectangular waveguide.

This formula means that if we change the (a) parameter in a rectangularwaveguide we will change its wavelength and in fact the phase for acertain length of waveguide. So if we place a piece of silicon in one ofthe waveguide walls and we change its dielectric constant from 11.8 toabove 100 in fact we will change the (a) dimension of the waveguidechanging its inside wavelength for a certain frequency.

The amount of phase change will depend then of the thickness on thesilicon piece, its position inside the waveguide, its length and thedielectric constant of the photoilluminated silicon that we willachieve. Special care must be taken to avoid losses in the waveguide ifwe try to achieve a big phase change in a short length and we push thewaveguide near cut off because the return losses of the device willincrease a lot.

If we analyse a rectangular waveguide with a piece of silicon in one ofthe walls, (see FIG. 12 a), we can conclude that happens a modepropagation that is very smilar to the normal rectangular waveguide. Infact, as can be seen in FIG. 8 b, the fundamental mode is very similarto the TE₁₀ of normal rectangular waveguide [Field Theory of GuidedWaves, Collin], this mode has the advantage that only a small amount ofthe field will travel inside the silicon insert, so the losses will below, and the cutoff frequency of this type of waveguide is lower than ina normal rectangular waveguide, (also an advantage, besides we must becareful with other modes that can appear at the higher frequencies ofthe band).

In FIG. 13 it can be seen the wavelength of a WR-28 waveguide with a 300μm thick piece of silicon in the wall of the waveguide under dark andilluminated conditions.

As shown in FIG. 13, the wavelength of a normal WR-28 waveguide and thesame waveguide filled with a 300 μm thick silicon in the wall under darkcondition is nearly the same. Upon illumination of the silicon, thedielectric constant changes inside it and produces a change in thewavelength and in fact in the phase. To achieve an efficient phasechange in a short device, the change of the dielectric constant of thesilicon by means of photoillumination must be high.

As an example, if we change the dielectric constant of the material from11.9 to 500, we need a length of 40 mm of silicon to achieve a total 360degrees phase change in the whole Ka band, but if we only reach adielectric constant of 100 a length of nearly 300 mm of silicon isneeded. So the device will be in the latter case not very practical ifthe aim is to obtain a 360° phase shift.

To reach a dielectric constant of 500 to allow an efficient and compactdevice over an area of 40×3.6 mm, means, see FIG. 5 that, the carrierconcentration must be above 10¹⁸ which is quite high. Such a highdensity plasma will not be reached with a normal light equipment and acostly equipment will be needed.

It can be seen from FIG. 14, that if a thicker silicon piece of 1 mmthickness is used, a length of 15 mm silicon that changes its dielectricconstant from 11.9 to 50 will suffer to achieve a 360° phase change inthe whole Ka band. This means a carrier concentration around 5·10¹⁶which is easily obtainable.

If a piece of a dielectric material is placed inside a rectangularwaveguide parallel to its dominant mode E field and spaced from aninside wall, simple finite-element simulation models can be solved toextract the modes of propagation inside that type of waveguide and itscharacteristics.

If we classify the modes of this type of waveguide for dark conditions,(FIGS. 15 and 16), we can see that there are three main modes inpropagation (WR-28 waveguide with 300 um thick silicon piece 0.85 mminside).

As shown in FIG. 15, the first mode in this type of waveguide, is a TE₂₀mode of a first type with part of its field inside the dielectric andpart of the field in the waveguide. The field intensity inside thedielectric is much lower (e.g. by a factor of 10 or more) than the fieldin the rest of the waveguide, so the losses are not high. Also this modecouples very well to the TE₁₀ of normal rectangular waveguide.

The second mode of this type of waveguide is a TE₁₀ mode of a secondtype that has its field concentrated inside the dielectric, (FIG. 12 a),so it will be very lossy for phase shift, but very effective asattenuator. The same principles can be applied to the third mode of thistype of waveguide, it is a TM₁₁ with its field concentrated inside thedielectric, (FIG. 12 b).

In FIG. 17 we can see a particular example of this type of waveguide.The wavelength of the two main modes is plotted against frequency for aWR-28 waveguide with a 300 μm thick silicon piece placed 0.85 mm insidethe waveguide, TM₁₁ mode is not plotted. IGS coupling efficiency to aTE₁₀ of normal rectangular waveguide is very low, so that it is suitableas an attenuator, not for phase shifting.

From the example of FIG. 17, we can see that the TE₂₀ mode (curves II,IV, VIII, IX, X), which seems to be the most beneficious mode reachescut off very soon for dark silicon. But when the ilumination over thesilicon increases, its cut-off frequency becomes lower. TE₁₀ mode is incut-off above a carrier concentration of 6·10¹⁴ (curve VII), so when theilumination increases, this lossy mode is no longer present, losses areheavily reduced, and the only mode that survives is the TE₂₀ that, asthe dielectric constant of the silicon increases, becomes more similarto the TE₁₀ of normal rectangular waveguide and its field inside thesilicon lowers a lot, (so lowering the losses of the component). Withdifferent waveguide dimensions and/or thickness of the dielectric piece,the carrier concentration above which the TE10 mode is in cut-off willbe different, but this effect will be useable by adjusting the intensityof light to place this mode (or other modes of the same type) in acut-off state.

So what is obtained with the example of FIG. 17 is:

-   -   a change in the wavelength inside the waveguide from 13 mm (TE₁₀        mode) to more than 25 mm (TE₂₀ mode) at 26.5 GHz changing the        amount of carriers from 10¹² to 10¹⁵ in the silicon piece    -   a change in wavelength at 35 GHz from 16 mm to 13 mm if we        assume only TE₂₀ mode    -   and a change in wavelength at 40 GHz from 11 mm to 9 mm assuming        only TE₂₀ mode

With this structure, a complete 360° phase shifter works in a frequencyrange from approximately 34 GHz to 40 GHz with a length of 44 mm andwith not a huge amount of light (10¹⁵ carriers per cubic centimeter).

At lower frequencies (less than 34 GHz) and in the dark state (noillumination), the travelling mode in the phase shifter is the TE₁₀ andwhen there is photoillumination the mode must change to the TE₂₀. TheTE₁₀ of the phase shifter couples badly to the TE₁₀ of a normalwaveguide and coupling losses are high in the two transitions. Besidesthe losses inherent to the power travelling inside the silicon for acertain length are high.

According to the invention, the piece of photo-responsive material maybe illuminated at the Brewster angle (or less), so that internalreflection occurs and all of the light is absorbed and propagates alongthe length of the piece of photo-responsive material. This will reducethe amount of light required for a given phase shift or attenuationlevel.

1. A tuneable phase shifter and/or attenuator comprising a waveguidehaving a channel and a piece of photo-responsive material (18) disposedwithin the waveguide along an internal wall of said channel, a lightsource disposed outside the waveguide to emit light through an aperture(30) of said internal wall to impinge on at least part of an outsidesurface of said piece of photo-responsive material (18).
 2. The tuneablephase shifter and/or attenuator as in claim 1, wherein thephoto-responsive material (18) is a photo-conductive material, e.g. Si,GaAs or Ge.
 3. The tuneable phase shifter and/or attenuator as in claim1 wherein at least the surface of the piece of photo-responsive materialfacing the aperture is pacified.
 4. The tuneable phase shifter and/orattenuator as in claim 3, wherein at least the surface of the piece ofphoto-responsive material facing the aperture has a coating of an epoxyresin.
 5. The tuneable phase shifter and/or attenuator as in claim 1,wherein at least part of the surface of the piece of photo-responsivematerial facing the aperture is covered with strips of reflectiveelements.
 6. The tuneable phase shifter and/or attenuator as in claim 5,wherein said strips form a grid.
 7. A tuneable phase shifter and/orattenuator comprising a waveguide having a channel and a piece ofphoto-responsive material disposed within the waveguide and spaced froman internal wall of said channel, and a light source to emit light toimpinge on at least part of a surface of said piece of photo-responsivematerial, the light source being adjustable to generate in the piece ofphoto-responsive material a carrier concentration between 10¹² cm⁻³ and10¹⁶ cm⁻³, to modify the real and imaginary part of the dielectricconstant of the photo-responsive material whereby at least one mode isgenerated that has part of its field inside the piece ofphoto-responsive material and part of its field in the waveguide wherebya phase shifter and/or attenuator that is dependant on the lightillumination is generated over a frequency range.
 8. A tuneable phaseshifter and/or attenuator as in claim 7, wherein said carrierconcentration is between 10¹⁴ cm⁻³ and 10¹⁶ cm⁻³.
 9. A tuneable phaseshifter and/or attenuator as in claim 7, wherein a said mode is of afirst type that has a field intensity inside the photo-responsivematerial layer that is small relative to the field in the channeloutside the photo-responsive material.
 10. A tuneable phase shifterand/or attenuator as in claim 9, wherein said mode of a first type isTE₂₀.
 11. A tuneable phase shifter and/or attenuator as in claim 7,wherein a said mode is of a second type that has a field intensityinside the photo-responsive material that is high relative to the fieldin the channel outside the photo-responsive material.
 12. A tuneablephase shifter and/or attenuator as in claim 7 wherein a said mode of thesecond type is TE₁₀ or TE₁₁.
 13. A tuneable phase shifter and/orattenuator as in claim 11, wherein the intensity of the light source isadjustable to place at least one of said modes of the second type in acut-off state.
 14. A tuneable phase shifter and/or attenuator as inclaim 1, wherein the illumination of the piece of photo-responsivematerial is carried out at an angle such that total internal reflectionoccurs.